Most ships currently engaged in the carriage of oil have a single hull or single skin usually referred to as a single bottom. If they run aground or collide with a submerged object or are involved in a collision with another vessel, they readily lose their cargo on the broaching of the hull, bottom or side, as was the case with the Exxon Valdez.
The provision of double hulls on tankers to avoid or at least reduce the loss of cargo following damage to the hull, bottom or side has been the subject of discussion for many years.
In general, the provision of a double hull has not been considered economical although the introduction of double hulls is most desirable from an antipollution point of view.
It is known to fit to an existing watercraft by way of a retrofit an additional hull internally of the main hull. In a known method an inner hull skin is fitted extending across the bottom of the ship and supported by transverse framing built up from the existing main hull bottom of the ship.
It is also known to build vessels with an additional hull or bottom, e.g. by building both hulls from sub-assemblies in sandwich-like form with inner and outer hull functions in each assembly, and then assembling these sub-assemblies on a building slip or in a dock.
Another method, more dramatic, is to cut off the existing single hull bottom structure and replace it with a newly constructed double bottom structure. This method has been applied to tankers with major grounding damage.
A retro fit of hull sides which is relatively common is the fitting of sponsons to the sides of a ship, which act essentially as buoyancy tanks and which are added to provide the ship with a greater water plane area and inertia and increase the carrying capacity of the ship. This is often done when the naval architects have made a design error, or when the owner or operator requires a change in the characteristics of the ship.
With known double bottomed watercraft, whether the double bottom was a feature of the craft initially or added internally as a retrofit, the spaces between the outer and inner hulls are left as void spaces. There is a risk that leakage of cargo into these spaces will occur, and so efficient monitoring systems are required for these spaces, as otherwise contamination of any ballast stored in the spaces may occur or there may be dangerous build up of explosive gases in the spaces.
Free flooding of void spaces in underwater structures is known in relation to oil platform jacket legs to prevent hydrostatic collapse of these deep water structures when it becomes uneconomic to build the legs to a size to withstand the hydrostatic pressure encountered at that depth.
There is one case common in ship design where a small compartment is free flooded and that is a sea water chest. A sea water chest is a small compartment having a volume of between 1 and 10 meters cubed, which is free flooded for pipe connections and which is used as a source of salt or fresh water for the convenience of the ship's piping systems as a water inlet. This compartment is always on the shell hull, and can be and sometimes is located between the outer and inner bottom.
It is well known to fit fenders to the sides of ships and boats to protect specific areas of the hull from contact damage. Fenders are typically corrugated with sections between 12 and 14 inches deep and are fitted to vessels, such as tugs, supply boats and ferries that are frequently involved in berthing and other operations in which contact with other vessels occurs. Fenders are fitted to the sides of the vessels usually above the water-line of the vessel and are to protect the vessel from damage by contact with docks or with other vessels. Fenders are fitted only to those selected parts of the vessel that come into contact with docks or other vessels.